This invention relates generally to systems for producing ion sensors, and more particularly, to a system which produces ion sensors which employ ion-responsive membranes formed of polymeric materials using screen printing procedures to achieve high reproducibility.
In order for an ion sensor to be commercially acceptable and successful, it must be possessed of qualities beyond electrochemical performance. In order for a sensor to be cost effective, it must be reproducible using mass production systems. Moreover, there must be common electrochemical response characteristics within the members of a batch fabricated group. If the sensors are not all substantially identical, they will each be characterized by different lifetimes and response characteristics, creating difficulties in the field, not the least of which is the added cost associated with recalibration of equipment whenever the sensor is changed. In general, polymeric membranes are in common use as transducers in solid-state chemical sensors, particularly because such membranes have high selectivity to the ion of interest and can be made selective to a wide range of ions using one or many readily available ionophores. One known technique for forming the membranes is solvent casting; a technique which originated with ion-selective electrode technology. In this approach, the membranes are cast by dissolving their components in an organic solvent, hand depositing the solution onto the sensor sites, and allowing the solvent to be removed by evaporation. In addition to being a rather tedious operation, particularly in view of the small size of the sensors, this production method yields very high losses. The thickness and shape of the membrane cannot be controlled, resulting in an unacceptable lack of sensor reproducibility.
These problems and concerns have been addressed in the prior art, but adequate solutions have not been found. For example, one known approach involves the use of a blank membrane solution to form a coating which conforms to the sensor. The membrane coating is then selectively doped over the multiple sensor sites. This technique suffers from the disadvantage that considerable hand work must be performed under a microscope. In addition, the ionophore will diffuse laterally over time. Moreover, electrical interference in the form of cross-talk through the membrane would limit the geometries and spacings between input pads to dimensions which are unacceptable for multisensing devices.
A further known system employs a lift-off method for patterning permselective membranes. This system requires the patterning of the silicon wafer with a positive photoresist, which results in the photoresist being removed in the areas where the-membrane is to be deposited. The dissolved membrane solution is spin coated onto the wafer and allowed to dry. The wafer is immersed in an ultrasonic solvent bath which removes the membrane-coated photoresist regions, resulting in a precisely patterned wafer. This method suffers from the disadvantage of exposing the membranes to organic lift-off solvents which may alter the electrochemical characteristics of the membranes. In addition, the range of thicknesses is limited to those which can be realized in photoresist. Moreover, in multisensing devices, cross-contamination in the ultrasonic lift-off bath can be a problem.
There is not currently available any suitable system for batch fabrication of solid-state ion-selective sensors which employ polymeric membranes. Such membranes can be formed of a variety of polymeric materials, such as poly(vinyl chloride), polyurethane, and silicone.
The use of polyurethane and silicone in the ion-selective membranes of chemical sensors is described in the parent applications enumerated hereinabove. These applications are all incorporated herein by reference.
It is, therefore, an object of this invention to provide a simple and economical system for batch fabrication of solid-state ion-selective sensors.
It is another object of this invention to provide a system for mass producing solid-state ion-selective sensors which employ ion-selective membranes formed of polymeric materials.
It is also an object of this invention to provide a system for batch fabrication of solid-state ion-selective sensors which results in high yield and with high uniformity between the respective sensors.
It is a further object of this invention to provide a substance-sensitive membrane system for a solid-state sensor which can be applied to a plurality of solid-state devices simultaneously using conventional techniques.
It is additionally an object of this invention to provide a substance-sensitive polymeric membrane system for a solid-state sensor which can be applied to a multiplicity of solid-state devices simultaneously using conventional integrated circuit manufacturing techniques.
It is yet a further object of this invention to provide a substance-sensitive membrane for use with a solid-state sensor which does not require a structural layer associated therewith to maintain communication between the membrane and a solid-state substrate.